1. Field of the Invention
The present invention relates generally to a noise eliminating circuit and more particularly to a circuit for eliminating a noise from a reproduced video signal from, for example, a video tape recorder (VTR) or the like.
2. Description of the Prior Art
FIG. 1 illustrates an example of a conventional noise eliminating circuit of a first-order configuration type for eliminating a noise from a reproduced video signal from, for example, a video tape recorder (VTR) or the like.
As FIG. 1 shows, a video signal Sv supplied to an input terminal 51 is supplied to a subtracter 52. This video signal Sv also is supplied to the subtracter 52 through a series circuit of a high-pass filter 53 and a limiter 54 forming a noise extracting circuit. The subtractor 52 serves to subtract from the video signal Sv low level high frequency signal components derived from the limiter 54. As a result a noise eliminated video signal is supplied to an output terminal 55 of the subtracter 52.
The conventional noise eliminating circuit shown in FIG. 1 is defective in that a noise at the edge portion of the video signal Sv cannot be eliminated. This defect is explained below. Let it now be assumed that the video signal Sv applied to the input terminal 51 has an edge portion as shown in FIG. 2A, and in which the reference letter N denotes a noise. The output signal from the high-pass filter 53 appears as shown in FIG. 2B and the output signal from the limiter 54 appears as shown in FIG. 2C. In FIG. 2B, TH1 and -TH1 represent the limiting levels. It will be seen that the level of the output signal from the high-pass filter 53 becomes high at the portion where the signal level of the video signal Sv changes greatly, so that the limiter 54 cannot effectively extract the noise N. Thus, the video signal developed at the output terminal 55 contains the noise N at its edge portion as shown in FIG. 2D.
FIG. 3 illustrates another example of a conventional noise eliminating circuit for eliminating a noise from a reproduced video signal for example, from a video tape recorder. This conventional noise eliminating circuit shown in FIG. 3 utilizes a line correlation of the video signal to eliminate a noise.
Referring to FIG. 3, a video signal Sv applied to an input terminal 61 is supplied to a subtracter 62. This video signal Sv also is supplied to a subtracter 63 which receives a noise reduced video signal from the subtracter 62 through a delay line 64 having a delay time of 1H. Here, H represents one horizontal scan period. The subtracter 63 serves to subtract the output signal of the delay line 64 from the video signal Sv to produce a signal formed of the noise component and a line non-correlation component therefrom. The output signal from the subtracter 63 is supplied to a limiter 65 to extract the noise component. The limiter 65 is a noise extracting circuit which only extracts a noise component with a relatively small amplitude. The thus extracted noise component is then supplied to the subtracter 62 where it is subtracted from the video signal Sv. Since the closed loop circuit averages the video signal over a plurality of horizontal lines, the noise reduced video signal is fed to an output terminal 66 at the output of the subtracter 62.
Because the noise eliminating circuit example shown in FIG. 3 utilizes the line correlation of the video signal, the circuit causes no defect if the present line and the preceding or following line have a correlation therebetween. If on the other hand they have no line correlation therebetween, a component other than the noise component is eliminated from the video signal Sv, and as a result the quality of a reproduced picture is deteriorated.
Japanese laid-open patent application gazette No. 57-185773 discloses a noise eliminating circuit of the kind shown in the example of FIG. 3.
As described above, the noise eliminating circuit shown in FIG. 1 is defective in that the noise at the edge portion of the video signal cannot be eliminated. On the other hand, the noise eliminating circuit shown in FIG. 3 is defective in that the quality of a reproduced picture will be deteriorated when there is no line correlation between the present line and the preceding or following line.